1. Field of the Invention
The present invention relates to an optical disc playback apparatus capable of judging disc type and a method of judging disc type in the optical disc playback apparatus.
2. Description of the Background Art
FIG. 16 shows focus error signal waveforms in a conventional optical disc playback apparatus. There are shown focus error signals when searching is performed for optical discs of different types by means of an optical pickup using a lens having a numerical aperture (NA) of 0.6, which is generally used for DVD playback. Specifically, a waveform 72 corresponds to a focus error signal when a CD is used, a waveform 73 corresponds to a focus error signal when a single-layer DVD (DVD-SL) is used, and a waveform 74 corresponds to a focus error signal when a double-layer DVD (DVD-DL) is used.
As is apparent from FIG. 16, the focus error signal 73 in the single-layer DVD has a large amplitude, characterized by having its peak (maximum voltage level) and bottom (minimum voltage level) each outputted once in the waveform. The focus error signal 74 in the double-layer DVD is characterized by having its peak and bottom each outputted twice in the waveform. The focus error signal 72 in the CD has an extremely small amplitude when the lens for a DVD having a numerical aperture of 0.6 is used, rendering the peak and bottom of the waveform indefinite.
FIG. 17 shows a structure of a twin lens optical pickup comprising two lenses having different numerical apertures. The twin lens pickup shown in the drawing comprises an objective lens 75 for a CD in general use (NA=0.45), an objective lens 76 for a DVD in general use (NA=0.6), a central shaft 77 of an actuator, an actuator base 78, a tracking coil 79, a focusing coil 80, a mirror 81, a laser diode 82, a half mirror 83 and a photodetector 84.
Laser outputted from the laser diode 82 is conveyed to the mirror 81 by the half mirror 83 and radiated onto an optical disc (not shown) through the objective lens 76 for a DVD. The laser reflected from the optical disc passes through the mirror 81 and the half mirror 83, and inputted into the photodetector 84.
Since a DVD and a CD are different in pit size of a recording signal and thickness from a surface of the optical disc to a recording layer, a focusing position and a spot diameter of laser light needs to be varied depending on the type of the optical disc in order for compatible playback between a DVD and a CD. Thus, an axial sliding and rotating method is employed in the twin lens optical pickup shown in FIG. 17, thereby switching the objective lens 75 for a CD and the objective lens 76 for a DVD. In this axial sliding and rotating method, the actuator base 78 is moved up and down along the central shaft 77 by driving the focusing coil 80 and is rotated about the central shaft 77 by driving the tracking coil 79.
FIG. 18 shows a structure of a disc judging circuit in the conventional optical disc playback apparatus. The optical disc playback apparatus shown in the drawing comprises a peak number counting circuit 90 and a judging circuit 91. The result of disc judgment is provided for a lens kick circuit 92, a servo parameter changing circuit 93 and a servo gain-up switching circuit 94. The lens kick circuit 92 has its output given to a driver 95 for driving the tracking coil 79.
Next, explanation will be given on the operation of the disc judging circuit shown in FIG. 18. A focus error signal obtained from the optical pickup is inputted into the peak number counting circuit 90. The peak number counting circuit 90 counts the number that peak and bottom appear in the focus error signal. When the inputted focus error signal has waveform with its peak and bottom each appearing once, the peak number is counted as one. The counted number is transmitted to the judging circuit 91. The judging circuit 91 detects the characteristics of focus error signals in various types of discs as shown in FIG. 16 based on an output signal indicative of the peak number outputted from the peak number counting circuit 90, thereby judging the disc type, whether a CD, a single-layer DVD or a double-layer DVD.
A signal indicative of the judgment result is transmitted to the lens kick circuit 92, the servo parameter changing circuit and the servo gain-up switching circuit 94. The lens kick circuit 92 drives the tracking actuator (tracking coil) 79 through the tracking actuator driver 95 to make a switch between the objective lens 75 for CDs and the objective lens 76 for DVDs. The servo parameter changing circuit 93 changes a servo parameter to one that is optimum for each disc.
Generally, in performing focus servo control, a servo gain is automatically adjusted during a constant operation, which allows a servo system to operate extremely stably. However, before the constant operation is started, for example, just after a focus is drawn in, the servo gain is not automatically adjusted. When there is a difference in a focus gain depending on a difference in reflectance of a disc, a servo may not be drawn in. Therefore, the servo gain-up switching circuit 94 sets an optimum servo gain-up for each disc based upon the result of the disc type judgment.
FIG. 19 shows waveforms indicating a measuring procedure at disc judgment and focus-on operation performed on the double-layer DVD. There is shown a voltage waveform of the focus actuator (focusing coil) 80 and a waveform of a focus error signal which are obtained at disc judgment. The reference character 96 indicates a focus actuator voltage waveform, and the reference character 97 indicates a focus error signal waveform, in a series of operations from disc judgment to focus-on. As can be seen from the drawing, the focus actuator is first moved up and down to count the number of focus error signals FS, thereby performing the disc judgment. Thereafter, the optical head is moved into an inner periphery, and the focus actuator is again moved up and down, so that the focus is drawn in.
FIG. 20 shows where and how an optical head is positioned at disc judgment and focus-on operation performed as shown in FIG. 19. The reference characters 98, 99, 100 and 101 indicate an optical disc, a spindle motor, an inner switch and an optical head, respectively. First, the optical head is positioned in an outer peripheral direction to perform the disc judgment. After the disc judgment is finished, the optical head is moved into an inner periphery so that the focus is drawn in.
The above described conventional disc judging method is based on the principle shown in FIG. 16 using the objective lens for a DVD, which is applicable only to a system using two objective lenses, one for a DVD and the other for a CD. A judging method different from the conventional one is required in a system using a pickup with a bifocal lens which is one lens having two focal points. After the focus is drawn in, it is necessary to make a switch of a tracking method (between the triple beam method and the DPD (phase difference) method) and to change the settings of the servo gain. Thus, judgment should be carried out in a step prior to tracking.
In the conventional method, there has been a possibility of making a misjudgment with variations in temperature in an apparatus, fluctuations in laser power due to adjustment variations in early stages of volume production and variations in disc reflectance. In addition, fingerprints and scratches on a disc and the like cause variations in equivalent reflectance, which also result in factors causing a misjudgment.
Further, when the laser power and the reflectance are increased, an unnecessary focus error signal to a surface of a disc substrate, not to an information surface, may also result in a misjudgment. Furthermore, when peak and bottom values are misdetected by signal noise, a misjudgment similarly occurs.